1. Field of the Invention
The present invention relates to a process for producing an aromatic carboxylic acid by a liquid-phase oxidation of an alkyl-group-substituted aromatic compound. More particularly, the present invention is concerned with the recovery of acetic acid which is used as a solvent in the aforesaid liquid-phase oxidation in said process.
2. Description of the Related Arts
A general process for producing an aromatic carboxylic acid such as terephthalic acid or isophthalic acid is a process in which an alkyl-group-substituted aromatic compound as the starting raw material is subjected to a liquid-phase oxidation by the use of molecular oxygen gas in acetic acid as a solvent in the presence of a catalyst. Since the acetic acid used as a solvent in the aforesaid process is diluted with water formed in the oxidation reaction, it is necessary to separate said water and return the acetic acid to the oxidation reactor for recycling through a distillation dehydration tower.
That is to say, said process obliges the acetic acid to be dehydrated into concentrated acetic acid in a dehydration tower for the purpose of reuse as the solvent, since water formed in the liquid-phase oxidation reaction is mixed in the water-containing acetic acid, for example, which is obtained by evaporating the reaction mother liquor produced by separating an aromatic carboxylic acid from the oxidation reaction product in the form of a slurry, or which is obtained by cooling condensing the exhaust gas coming out from an oxidation reactor.
The foregoing dehydration tower is charged with the water-containing acetic acid at an intermediate stage thereof, distills away water at the top portion, and discharges concentrated acetic acid at the bottom portion. However, since the water distilled away at the top portion of the tower is discharged to the outside of the system, the concentration of the acetic acid contained in the discharged water must be suppressed to a minimum level. In order to respond to such a demand, it is obliged to construct a dehydration tower having a large number of distillation trays and also to consume a great deal of energy.
In order to suppress at least part of the factors of increasing these costs involved in the recovery of acetic acid, there is proposed an azeotropic dehydration process which employs n-butyl acetate or the like as an azeotropic agent, for example, there is disclosed a method for separating acetic acid from water through azeotropic distillation by the use of an entrainer (azeotropic agent) in Japanese Patent Publication No. 31091/1986 (Sho-61).
In the meanwhile, a long-term continuous operation was carried out by the present inventors for an azeotropic dehydration pilot plant of acetic acid by using n-butyl acetate as an azeotropic agent, and using, as a starting raw material, the mixture of acetic acid and water containing oxidation reaction water which mixture was formed from commercially operated plants for producing terephthalic acid and isophthalic acid. As a result, there was observed a phenomenon such that the separation efficiency of acetic acid from azeotropic components (n-butyl acetate/water) gradually decreases with the lapse of time and day from the start of running the azeotropic dehydration pilot plant (refer to Comparative Examples 1&2).